Sulphation of secondary alcohols

ABSTRACT

A process for sulphating secondary alcohols having from eight to 26 carbon atoms with free SO3 in which insufficient SO3 is employed to effect complete sulphation of the alcohol the product is neutralised and the unreacted alcohol separated from the desired sulphate product, preferably by solvent extraction. The unreacted alcohol is re-cycled to the first stage in the process.

United States Patent Sowerby 14 1 Feb. 11, 1975 SULPHATION OF SECONDARY ALCOHOLS 3.133.244 i840 glillsnfi 560/459 ,2 .649 41 t t' 60459 [751 inventor swerbyr 3.232.976 H1966 260459 England 3,270,038 8/l966 Marshall et al. 260/459 [731 Assigneel Ltd-3 FOREIGN PATENTS OR APPLICATIONS Brmmg Eng 1.066.505 4/1967 Great Britain 260/459 [22] Filed: Apr. 16, 1968 1 A N I: 721 7 Primary Examiner-Joseph E. Evans [2 1 pp] 0 6 0 Attorney, Agent. or Firm-Herbert H. Goodman [30] Foreign Application Priority Data I S7] ABSTRACT Apr. 25, 1967 Great Britam 19025/67 A process for Sulphating Secondary alcohols having [52] U S Cl 260/459 252/1310 13 from eight to 26 carbon atoms with free 50 in which [Sll Irit Cl llllllllllllllllllll H C07c U insufficient S0 is employed to effect complete sul- [58] Fie'ld 260/459 phation of the alcohol the product is neutralised and the unreacted alcohol separated from the desired sulphate product, preferably by solvent extraction. The [56] g ga zg gii unreacted alcohol is re-cycled to the first stage in the process. 2,l6l,857 6/1939 Davidson .1 260/459 2,178,786 11/1939 Fasce 260/459 17 Clam, l zs Flgure A L C OHOL ISOPROPYL J 1 A ALCOHOL BA SE r\ r\ 0R GAN/C mm SOL VEN T V AQUEOUS SOLUTION 0F ALKYL SULPHATE SALT 1 SULPHATION OF SECONDARY ALCOHOLS This invention relates to a new method for using sulphur trioxide as a sulphating agent for saturated secondary aliphatic alcohols to obtain alkyl sulphates useful asdetergents.

Hitherto there has been little large scale use of sul phur trioxide in a gaseous form for the sulphation of long chain secondary alcohols because of their tendency to undergo dehydration during the process to form olefins and to char the product. This resulted in an unsatisfactory yield of alkyl sulphates which moreover were highly coloured and it has therefore been believed that the use of gaseous sulphur trioxide for the sulphation of secondary aliphatic alcohols was to be avoided.

In order to overcome these disadvantages it has been proposed to carry out the sulphation reaction very rapidly so as to cut down the length of time during which the product was exposed to the reaction conditions. It has also been stated that dehydration and charring of the product does not begin until more than a certain percentage of the alcohol has been sulphated, the particular percentage in a given instance being stated to depend upon the conditions employed. A proposal based on these ideas had been made to carry out the sulphation comparatively slowly using conventional methods until this point is reached and then to transfer the reaction mixture to another vessel, suitable for use in high speed reaction, and to complete the reaction rapidly. Such a process would require the transfer of the reaction mixture from the vessel to another and the use of complicated and expensive cooling means associated with rapid reaction, such means being used only intermittently and therefore inefficiently.

We have now found that we may completely dispense with a stage in which a high reaction rate is required by carrying out the sulphation to the desired degree and then neutralising the reaction product, a mixture of a salt of the alkyl sulphate and unreacted alcohol being obtained. The unreacted alcohol may be separated from the mixture for re-use, preferably by solvent extraction into an organic solvent, the removal of which, for example by distillation liberates the alcohol for reuse if desired.

Accordingly, the present invention provides a process for-the production of mono-alkyl sulphate salts from saturated secondary aliphatic alcohols having at least eight carbon atoms and free sulphur trioxide, which comprises: (l) adding to the secondary alcohol free sulphur trioxide in an amount sufficient to react with from to 80% of the secondary alcohol present but insufficient to cause substantial dehydration of the alcohol or charring in the reaction mixture under the conditions employed: (2) neutralising the reaction mixture so formed with a base; and (3) separating the unreacted alcohol from the salt of the alkyl sulphate formed. Preferably the unreacted alcohol is separated by a process of solvent extraction and the unreacted alcohol will then normally be released from the solvent, for example by distillation of the solvent, and recycled for a further reaction with sulphur trioxide.

Accordingly, a preferred feature of the invention is the provision of a continuous process for the production of mono-alkyl sulphate salts from saturated secondary aliphatic alcohols having at least eight carbon atoms and sulphur trioxide, which comprises: (1) adding to the secondary alcohol free sulphur trioxide in an amount sufficient to react with from 20 to 80 percent of the secondary alcohol present but insufficient to cause substantial dehydration of the alcohol or charring of the reaction mixture under the conditions employed; (2) neutralising the reaction mixture so formed with an aqueous solution of a base; (3) separating the unreacted alcohol from the salt of the alkyl sulphate formed, preferably by a process of solvent extraction; and (4) recycling the separated unreacted alcohol to stage (1) of the process where it is mixed with a further quantity of the alcohol.

The process of the invention is suitable for the sulphation of saturated secondary aliphatic alcohols having from eight to 26 carbon atoms preferably those having from 10 to 20 carbon atoms and particularly so for those having from 13 to l8 carbon atoms. The process may also be used for sulphating mixtures of such alcohols. The alcohols used may have inert substituents, but it is much preferred that there is no steric hinderence of the hydroxyl group. Alcohols for use in the process of the invention are conveniently made by the oxidation of paraffins obtained by fractional distillation of petroleum. Such alcohols have: unbranched chains wherein the hydroxyl group is preferably in the 2- or 3position, for instance 2- or 3-decanols, -dodecanols, -tridecanols, -hexadecanols, octadecanols, and -duodecanols and mixture thereof.

In carrying out the process of the invention it is preferred to add to the secondary alcohol as much sulphur trioxide as possible commensurate with the avoidance of the undesired side reactions. Thus normally is is preferred to use an amount of the free sulphur trioxide suf ficient to react with from about 50 to percent, more usually 50 to 60 percent, of the alcohol present when operating at ambient temperatures.

The temperature at which the addition of sulphur trioxide to the secondary alcohol is carried out will normally be as low as is possible to maintain the alcohol as a liquid, this allows the maximum possible amount of sulphur trioxide that will not cause dehydration of the alcohol to be added. It is preferred that the temperatures used should not be very much greater than ambient, desirably not greater than 50C, and we have found that for sulphating a mixture of alcohols having from 13 to 17 carbon atoms a temperature of about 35C is satisfactory. With alcohols of shorter chain lengths lower temperatures, say less than 10C, can be used with advantage. Thus it may be desirable to fit a cooling jacket or some internal cooling system to the reactor used.

Although the sulphur trioxide used may be dissolved in a low boiling solvent, such as carbon tetrachloride, methylene dichloride, low boiling petroleum ethers or liquid sulphur dioxide, or if refrigerated apparatus is used, be in the form of pure liquid sulphur trioxide, the use of gaseous sulphur trioxide is preferred. The sulphur trioxide may be added in a gaseous form either as pure sulphur trioxide vapour or more preferably admixed with an inert gas diluent. Suitable inert gas diluents include dry air, nitrogen, sulphur dioxide, carbon dioxide and argon. If the pure gas is used it is then highly preferred that the apparatus be kept at a subatmospheric pressure, at say less than 3 p.s.i., preferably in the region of from 0.5 to 2.0 p.s.i. If gaseous sulphur trioxide is used in an inert gas diluent, the diluent can be introduced at a pressure of from 0.5 to 20 p.s.i.g. to suit the convenience of the operator and the sulphur trioxide desirably comprises from 1 to 15 percent by volume preferably from 5 to percent volume of the gas mixture.

The reaction vessel used for this first stage of the process may be any conventional sulphation vessel appropriate to the use of liquid or gaseous sulphur trioxide as the case may be.

The product obtained from the partial sulphation of the secondary alcohol is mainly a mixture of alkyl sulphate and unreacted secondary alcohol. lf gaseous sulphur trioxide is used the product should be degassed at this stage to remove any sulphur trioxide remaining. This can conveniently be done by use of a centrifuge.

In the second stage of the process of the invention this product is neutralised with a base to convert the alkyl sulphate to its salt with the base used. It has been found convenient to carry out the neutralisation in a convergent-divergent type of neutralising apparatus, for example to carry out the neutralisation by means of the process described in British Patent No. 1,052,976. It has also been found that under some conditions there may a tendency for the pH of the solution to fall to less than 7. Should such a pH drift occur it may be corrected by the use of a slight molar excess of base in the neutralisation process. The base used will normally be an aqueous solution of an alkali, amine, or alkanolamine which is inert to the unreacted alcohol. It has been found that it is preferred to employ a solvent for the base in which the alkyl sulphate salt formed is also soluble. For example, the use of aqueous methanol, ethanol or isopropanol is often more preferred than the use of water itself. Such solvent or solvent mixture may be added to the reaction mixture at the same time as the base or subsequently thereto, in either case the presence of the solvent aids the separation of the unreacted alcohol from the neutralised alkyl sulphate. The base may also be used in the absence of a solvent, although this is not normally preferred. However, in the case where ammonia is used then this may with advantage be blown through the mixture as a gas.

The alkali will often be sodium hydroxide, but potassium or lithium hydroxide may be used in special circumstances, for example in the preparation of detergents useful in carpet shampoos, as also may dilute solutions of calcium hydroxide. The alkanolammonium alkyl sulphates find particular application in shampoos and other similar light-duty detergent compositions and thus there may be used as the base monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine or tri-isoporpanolamine. It is often desired to convert the alkyl sulphate to its sodium salt and this can be brought about by adding a strong aqueous caustic soda solution, often derived from soda ash, of about 25 to 60 percent by weight, preferably from 35 to 45 percent by weight, to the mixture of alkyl sulphate and unreacted alcohol. If a solvent were used for the sulphur trioxide it would be removed for example by distillation at this stage.

The unreacted alcohol can be removed by extraction into an organic solvent. The solvents to be used are those which are immiscible with water and will dissolve the unreacted alcohol but not the alkyl sulphate salt. Such solvents should be those of low boiling point so that they may be easily separated from the unreacted alcohol by distillation. Solvents of particular use in the process of the invention are low boiling point petroleum ethers that is those having a boiling point in the range 40 to C. Other solvents which may be used include benzene, alkyl benzenes, and diethyl ether.

The solvent extractions may be carried out in any convenient fashion. For example if continuous extraction is required a countercurrent system may be employed, whereas if a batch process is operated it is possible to bring about the extraction by simpler methods of agitation to mix the two phases and then allowing them to separate by settlement.

The organic solvent can be stripped from the unreacted alcohol by conventional distillation means, often using reduced pressure. The solvent may be consensed for re-use and the unreacted alcohol returned to the sulphation reaction vessel for re-use in stage (1 of the process after mixing with a further quantity of the alcohol.

If the solution of base used contains an organic solvent for the alkyl sulphate salt, then this is usually removed from the aqueous phase remaining after the solvent extraction of the unreacted alcohol, for example by distillation though depending on the detergent formulation in which the alkyl sulphate is to find use, and the nature of such a solvent, it may be allowed to remain associated with the alkyl sulphate.

The overall process may be carried out on either a batch or on a continuous basis, the latter normally being preferred for commercial reasons.

This process yields as its product an aqueous solution of a salt of an alkyl sulphate. This salt may then be sold either as a concentrated solution, optionally mixed with some other surface-active agent or agents or other adjuvants for detergent compositions of various kinds. It may also be sold in a flake form after drum drying, or a powder form after spary drying, the drying having been carried out either on the solution of the salt itself or on the salt in admixture with some other surface active agent or agents suitable for use in a detergent optionally together with other adjuvants.

The process is illustrated in its preferred continuous form with reference to the accompanying flow sheet.

Sulphur trioxide gas and liquid alcohol are passed into the reactor 1, the proportion of sulphur trioxide to alcohol being controlled to the amount that is required to bring about the desired degree of sulphation. The product from this reaction is passed into the neutraliser 2 where it is contacted with the calculated amount of base and also with an aqueous alcohol mixture. The reaction mixture is then passed to a solvent extractor 3 where it is contacted with an organic solvent in which the unreacted alcohol, but not the alkyl sulphate, is soluble. After separation of the phases the aqueous phase, which contains the alkyl sulphate salt, is then passed to a distillation column 4 where the solvent alcohol added to assist the solubility of the alkyl sulphate salt in water is stripped from the aqueous solution of alkyl sulphate salt and recycled to the neutraliser 2. The aqueous so lution of the salt is then removed for concentration or drying. The extracted phase taken from the extractor 3 is passed to a distillation column 5 wherein the solvent is stripped from the unreacted alcohol and recycled to the solvent extractor 3 the unreacted alcohol being returned to supplement the intake of alcohol into the reactor 1.

The invention is further illustrated by the following examples:

EXAMPLE 1 A batch reactor was charged with 1,000 parts by weight of a commercial C secondary alcohol obtained from the oxidation of normal paraffms. The sample used had an iodine value of 5.0. To this sample was added, with efficient agitation, 55 percent of the theoretical amount of sulphur trioxide needed to react with all of the alcohol used. The sulphur trioxide was in the form ofa 5 percent by volume mixture with dry air. The sulphur trioxide-air mixture was introduced to the reactor through a sparge pipe over a period of 70 minutes and the reaction temperature was controlled in the range of 35C by circulating coolant through the jacket of the reactor. Upon completion of the addition of sulphur trioxide the reaction product was immediately neutralised with a mixture of aqueous caustic soda and isopropyl alcohol to give a 48 percent yield of the sodium salt of sulphated C, C secondary alcohols. The unreacted alcohol was recovered from the reaction product mixture by extraction with petroleum ether and this was then distilled to reclaim the alcohol. lsopropyl alcohol was distilled from the aqueous solution of sulphated secondary alcohols to yield as product a pale yellow aqueous solution containing percent of the sodium salt of sulphated C secondary alcohols, 0.65 percent of sodium alkenyl sulphonate and 0.3% of unreacted organic matter.

EXAMPLE 2 A mixture of 500 gms. of the unreacted secondary alcohol recovered from Example 1 having an iodine value of 6.4, and 500 gms. of fresh C, .C, secondary alcohol having an iodine value of 5.0, was sulphated with 55% of the stoichiometric proportion, of sulphur trioxide under conditions identical to those employed in Example I, and neutralised with a mixture of aqueous caustic soda and isopropyl alcohol to give a 48 percent yield of the sodium salt of sulphated secondary alcohols. The unreacted secondary alcohol was recovered by extraction with petroleum ether and the isopropyl alcohol was distilled from the surface active solution to obtain, as product, a pale yellow aqueous solution containing 39.6 percent of the sodium salt of sulphated C C, secondary alcohols, 0.4 percent of sodium alkenyl sulphonate and 0.28 percent of unreacted alcohol.

EXAMPLE 3 A mixture of 500 gms. of unreacted secondary alcohols recovered from Example 2 having an iodine value of 7.0 and 500 gms. of fresh C C secondary alcohols having an iodine value of 5.0, was sulphated with 55 percent of the stoichiometric proportion of sulphur trioxide under conditions identical to those employed in Example 1, and neutralised with a mixture of aqueous caustic soda and isopropyl alcohol to give a 5l percent yield of the sodium salt of sulphated C -C secondary alcohols. The unreacted secondary alcohol, recovered by extraction with petroleum ether, had an iodine value of 6.7. The isopropyl alcohol was distilled from the surface active solution to obtain, as product, a pale yellow aqueous solution containing 4l .5 percent of the sodium salt of sulphated secondary alcohols, 0.8 percent of sodium alkenyl sulphonate and 0.35 percent of unreacted organic matter.

As comparative tests the following experiments were carried out using the amount of sulphur trioxide theoretically required to react with all of the secondary alcohol: Comparative Experiments EXPERIMENT l A commercial C secondary alcohol having an i0 dine value of 5.0 was sulphated in a batch reactor at a temperature of 35C with the amount of sulphur trioxide theoretically required to react with all of the alcohol, in the form of a 5 vol percent mixture with a dry air stream. After neutralising the reaction product with aqueous caustic soda a 52 percent yield of alkyl sulphate, calculated on the starting weight of alcohol, was obtained. The unreacted organic matter, which was recovered from the neutral product by extraction with petrol, had an iodine value of 30, showing that excessive dehydration of the secondary alcohol had occurred during sulphation. The recovered unreacted organic matter was saponified by heating it with aqueous caustic soda at C for 6 hours and then extracted again with petrol, to recover the unsaponifiable matter. The aqueous layer from this second extraction was found to contain sodium alkenyl sulphonate formed by the dehydration and sulphonation of 16.3 percent of the starting alcohol. Thus, under the reaction conditions employed, 68.3 percent of the secondary alcohol had reacted to yield a mixture comprising 76 percent of alkyl sulphates and 24 percent of alkenyl sulphonates.

EXPERIMENT 2 The unsaponifiable residue recovered in Experiment 1 was reacted again with the amount of sulphur trioxide theoretically required to react with all of the secondary alcohol, in the form of a 5 vol percent mixture with dry air and the reaction mixture neutralised with aqueous caustic soda to give a product so dark in colour that all attempts to bleach it were ineffective. The active ingredient in the product, after saponification at 80C for 8 hours, contained 64.8 percent of alkenyl sulphonates and only 35.2 percent of alkyl sulphates. The weight of unreacted organic matter was equivalent to 22 percent of the weight of unsaponifiable residue from Experiment 1.

We claim:

1. A process for the production of monoalkyl sulphate salts from secondary alkanols having from 8 to 26 carbon atoms by reaction with free sulphur trioxide comprising (1) adding to the secondary alkanol free sulphur trioxide in an amount sufficient to react with from 20 to 80 percent ofthe secondary alkanol present, the reaction being carried out at a temperature not greater than 50C, (2) neutralising the reaction mixture so formed with an aqueous solution of a base selected from the group consisting of aqueous solutions of alkalis, and amines, which is inert to the unreacted alkanol; (3) separating the unreacted alkanol from the salt of the alkyl sulphate formed by contacting said unreacted alkanol and said salt of the said alkyl sulphate formed with a solvent which is immiscible with water and which will dissolve said unreacted alkanol and will not dissolve said salt of the alkyl sulphate; and (4) recycling the unreacted alkanol separated in stage (3) of the process to stage (I 2. A process as claimed in claim 1 wherein said amine is an alkanolamine.

3. A process as claimed in claim 1 wherein the amount of free sulphur trioxide employed is sufficient to react with from about 50 to about 70 percent of the alkanol present.

4. A process as claimed in claim 3 wherein the sulphur trioxide is employed in the form of a gas.

5. A process as claimed in claim 4 wherein the sulphur trioxide is mixed with an inert gas diluent.

6. A process as claimed in claim 3 wherein the product of the first stage of the process is neutralised by a solution of an alkali metal hydroxide.

7. A process as claimed in claim 3 wherein the product of the first stage of the process is neutralised by an alkanolamine.

8. A process as claimed in claim 1 wherein the solvent employed is a mixture of low boiling petroleum ethers.

9. A process as claimed in claim 1 wherein the separation operation (3) is carried out by means of a counter current extractor.

10. A process as claimed in claim 1 wherein an amount of lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, or alkanolamine, sufficient to react with all of the sulphonic acid product present, is added to said reaction mixture to neutralise said reaction mixture at stage (2) of the process of claim 1.

11. A process for the production of monoalkyl sulphate salts from secondary alkanols having from to 20 carbon atoms by reaction with free sulphur trioxide comprising (1) adding to the secondary alkanol free sulphur trioxide in an amount sufficient to react with from 50 to percent of the secondary alkanol present, the reaction being carried out at a temperature not greater than 50C, (2) adding sufficient lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, or alkanolamine to react with all of the sulphonic acid product present in said reaction mixture and form the corresponding salt of the alkyl sulphate; (3) separating the unreacted alkanol from said salt of the alkyl sulphate formed; and (4) recycling the unreacted alkanol separated in stage (3) of the process to stage (1).

12. A process as claimed in claim ll wherein the salt of the alkyl sulphate is separated from the unreacted alkanol by means ofa solvent extraction separation employing a solvent selected from the group consisting of low boiling petroleum ethers, benzene, alkyl benzenes, and diethyl ether.

13. A process according to claim 12 wherein said secondary alkanol has from 13 to l8 carbon atoms.

14. A process according to claim 13 wherein the sulphur trioxide is admixed with an inert gas diluent in an amount of between about 5 and 10 percent of the volume of the gas mixture.

15. A process as claimed in claim 14 wherein the solvent employed is a mixture of low boiling petroleum ethers.

16. A process according to claim 15 wherein said reaction is carried out at a temperature of about 35C.

17. A process according to claim 16 wherein said free sulphur trioxide is in an amount sufficient to react with from 50 to 60 percent of said secondary alkanol.

TJNTTTD STATES PATENT OFFICE CTEMCATE CF CQRECTIUN Patent N D d February 1].,

Inventor(s) Austen E. SOWERBY It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 32, change "is", first occurrence, to

Column 3, line 50, correct the spelling of "tri-isopro panolamine";

Column 4, line 35, correct the spelling of "spray";

Column 7, line 1, replace "1" with -l0-.

a'gned and ealed this A nest:

RUTH C. MAON C. MARSHALL DANN 1 08! "8 ff ('mmnissimwr uflurz'nlx and Trademarks 

1. A PROCESS FOR THE PRODUCTION OF MONOALKYL SULPHATE SALTS FROM SECONDARY ALKANOLS HAVING FROM 8 TO 26 CARBON ATOMS BY REACTION WITH FREE SULPHUR TRIOXIDE COMPRISING (1) ADDING TO THE SECONDARY ALKANOL FREE SULPHUR TRIOXIDE IN AN AMOUNT SUFFICIENT TO REACT WITH FROM 20 TO 80 PERCENT OF THE SECONDARY ALKANOL PRESENT, THE REACTION BEING CARRIED OUT AT A TEMPERATURE NOT GREATER THAN 50*C, (2) NEUTRALISING THE REACTION MIXTURE SO FORMED WITH AN AQUEOUS SOLUTION OF A BASE SELECTED FROM THE GROUP CONSISTNG OF AQUEOUS SOLUTIONS OF ALKALIS, AND AMINES, WHICH IS INERT TO THE UNREACTED ALKANOL, (3) SEPARATING THE UNREACTED ALKANOL FROM THE SALT OF THE ALKYL SULPHATE FORMED BY CONTACTING SAID UNREACTED ALKANOL AND SAID SALT OF THE SAID ALKYL SULPHATE FORMED WITH A SOLVENT WHICH IS IMMISCIBLE WITH WATER AND WHICH WILL DISSOLVE SAID UNREACTED ALKANOL AND WILL NOT DISSOLVE SAID SALT OF THE ALKYL SULPHATE; AND (4) RECYCLING THE UNREACTED ALKANOL SEPARATED IN STAG (3) OF THE PROCESS TO STAGE (1).
 2. A process as claimed in claim 1 wherein said amine is an alkanolamine.
 3. A process as claimed in claim 1 wherein the amount of free sulphur trioxide employed is sufficient to react with from about 50 to about 70 percent of the alkanol present.
 4. A process as claimed in claim 3 wherein the sulphur trioxide is employed in the form of a gas.
 5. A process as claimed in claim 4 wherein the sulphur trioxide is mixed with an inert gas diluent.
 6. A process as claimed in claim 3 wherein the product of the first stage of the process is neutralised by a solution of an alkali metal hydroxide.
 7. A process as claimed in claim 3 wherein the product of the first stage of the process is neutralised by an alkanolamine.
 8. A process as claimed in claim 1 wherein the solvent employed is a mixture of low boiling petroleum ethers.
 9. A process as claimed in claim 1 wherein the separation operation (3) is carried out by means of a counter current extractor.
 10. A process as claimed in claim 1 wherein an amount of lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, or alkanolamine, sufficient to react with all of the sulphonic acid product present, is added to said reaction mixture to neutralise said reaction mixture at stage (2) of the process of claim
 1. 11. A process for the production of monoalkyl sulphate salts from secondary alkanols having from 10 to 20 carbon atoms by reaction with free sulphur trioxide comprising (1) adding to the secondary alkanol free sulphur trioxide in an amount sufficient to react with from 50 to 70 percent of the secondary alkanol present, the reaction being carried out at a temperature not greater than 50*C, (2) adding sufficient lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, or alkanolamine to react with all of the sulphonic acid product present in said reaction mixture and form the corresponding salt of the alkyl sulphate; (3) separating the unreacted alkanol from said salt of the alkyl sulphate formed; and (4) recycling the unreacted alkanol separated in stage (3) of the process to stage (1).
 12. A process as claimed in claim 11 wherein the salt of the alkyl sulphate is separated from the unreacted alkanol by means of a solvent extraction separation employing a solvent selected from the group consisting of low boiling petroleum ethers, benzene, alkyl benzenes, and diethyl ether.
 13. A process according to claim 12 wherein said secondary alkanol has from 13 to 18 carbon atoms.
 14. A process according to claim 13 wherein the sulphur trioxide is admixed with an inert gas diluent in an amount of between about 5 and 10 percent of the volume of the gas mixture.
 15. A process as claimed in claim 14 wherein the solvent employed is a mixture of low boiling petroleum ethers.
 16. A process according to claim 15 wherein said reaction is carried out at a temperature of about 35*C.
 17. A process according to claim 16 wherein said free sulphur trioxide is in an amount sufficient to react with from 50 to 60 percent of said secondary alkanol. 